The invention relates to a control device for the mechanical or thermal adjustment of the nozzle of an extruder for elongate products, having a wall thickness measuring device having at least one which reverses over the width of the elongate product or rotates around the periphery thereof and from whose measured value the controlled quantity is derived.
With the object of saving material, in the extrusion of strips or tubes an attempt is made to keep wall thickness as little as possible above the minimum wall thickness. Wall thickness fluctuations may occur both over the width and periphery of the elongate product and also in the longitudinal direction.
It is characteristic of thickness fluctuations in the longitudinal direction that the wall thickness is constant over the strip width or tube periphery in individual planes lying perpendicular to the axis of the elongate product. Such thickness fluctuations may be caused by temperature changes in the product, changes in the speed of the extruder screw, or by a change in the drawing-off speed. Such thickness fluctuations in the longitudinal direction can be levelled by suitable control steps.
Different wall thicknesses in the peripheral direction or over the width of the elongate product can have various causes. One frequent cause is the geometry of the extruder nozzle or different temperatures of the product over nozzle width or periphery. Such wall thickness fluctuations can be compensated by control steps at the nozzle. It is known to perform this either mechanically or thermally.
One difficulty in controlling wall thickness is therefore that the wall thickness is affected by nozzle adjustment, the temperature of the material to be extruded, the drawing-off speed and the screw speed. Thickness fluctuations in the longitudinal direction are superposed on thickness fluctuations in the peripheral direction. This means that the measured values of the reversing or rotating ultrasonic measuring head can be used as a controlled condition only within certain limits.
It is an object of the invention to provide a control device for the mechanical and/or thermal adjustment of an extruder nozzle, by means of which wall thickness fluctuations can be more satisfactorily levelled over the width and periphery of the elongate product.
This problem is solved according to the invention in a control device of the kind specified by at least one further stationary ultrasonic measuring head delivering its measured value to an evaluating unit which so links said measured value with the measured value of the reversing or rotating ultrasonic measuring head that the evaluating unit forms for the control of the wall thickness in the peripheral direction a signal which is free from superposed measured values of the stationary ultrasonic measuring head for exclusive wall thickness fluctuations in the longitudinal direction of the longitudinal product.
The control device according to the invention takes into account the trend of wall thickness fluctuations in the longitudinal direction by including the measured values of the stationary measuring head in the formation of the controller output, so that a rapid and high precision reaction is possible to wall thickness fluctuations over the width or periphery of the elongate product which can be levelled at the nozzle. It is true that it is also known to measure the wall thickness via a number of ultrasonic measuring heads distributed stationary over the periphery or width of the elongate product, but this fails to give a complete picture of the wall thickness over the total periphery or the whole width, since for practical reasons or reasons of space it is impossible to dispose the heads very close to one another. However, even if that were possible, the result would still not be any controlled variable of comparable quality, which is free from changes in thickness in the longitudinal direction.